arXiv subjects

A. D'Aì

Publications and source records attributed to A. D'Aì.

At least 19 recordsLinked to original sources

arXiv2026

Detection of kilosecond hard lags in the new pulsating ULX candidate NGC 7456 ULX-1

Context. Ultraluminous X-ray sources (ULXs) are thought to be powered, in many cases, by super-Eddington accretion onto compact objects. While soft X-ray lags have been detected in several ULXs, hard lags remain rare and poorly understood. Aims. We investigate the temporal and energy dependence of X-ray lags in NGC 7456 ULX-1 to constrain their physical origin and probe the super-Eddington accretion flow. Methods. We analyzed the two deepest XMM-Newton observations, taken in 2018 and 2023. Hard (1-10 keV) and soft (0.3-1 keV) light curves were cross-correlated using adaptive-binning techniques optimized for Poissonian low-count data. We measured lags over consecutive 10 ks intervals and investigated their energy dependence. Spectra were modeled using thermal and Comptonization models. Results. We detect significant hard X-ray lags in both observations, with the hard emission delayed by $\sim 10^3$ s during phases of rapid flux variability. The delays are primarily driven by the lowest-energy photons. Spectral modeling indicates a Comptonization-dominated flow comprising a cooler, extended outer region and a hotter, compact inner flow embedded in an optically thick wind. We interpret the delays as the combined effect of inward propagation of accretion-rate fluctuations and photon diffusion within the dense outflow. Fluctuations first enhance the soft-emitting outer regions and then propagate toward the hotter inner flow, where photons undergo stronger Comptonization before escaping with a kilosecond delay. The small inferred inner emitting radius disfavors an intermediate-mass black hole accretor. Conclusions. The sign, amplitude, and energy dependence of the delays disfavor standard reverberation. Propagation-driven variability coupled with radiative transfer in optically thick winds appears to play a major role in shaping the timing properties of super-Eddington accretion flows.

astro-ph.HE↗

arXiv2026

Fingerprints of thermal Comptonization in accreting neutron stars. Plasma-vacuum interplay in cyclotron lines and polarisation

X-ray emission from accreting, strongly magnetised neutron stars and its pulse-phase variability probe their magnetic-field geometry, spin orientation, and emission processes. Whether the radiation emerges mainly from a hot spot or column, and whether its properties are shaped by bulk or thermal Comptonization, remain debated across luminosity regimes. We aim to disentangle intrinsic emission from visibility effects and identify observables characteristic of thermal Comptonization in hot spots and columns. We therefore derived energy-dependent beam patterns and observable signatures without assigning the model to a luminosity regime, focusing on cyclotron-resonance and polarisation effects as tracers of anisotropy. To do so, we computed angle-dependent polarised broadband spectra, including the fundamental cyclotron line, for a homogeneous, self-emitting, magnetised Comptonizing plasma over a broad parameter range. Accounting for light bending and projection, we obtained phase-dependent fluxes for different geometries and, for hot spots, observed linear polarisation. The beam patterns evolve with energy, driving pulse-profile changes. Near the cyclotron resonance, plasma-vacuum interplay produces a narrow central beam and side petals. Their visibility creates geometry-dependent dips, bumps, and M- and W-shaped structures in hot-spot pulsed fraction spectra, but only dips and bumps for columns. Thermally Comptonized cyclotron lines do not reliably trace plasma temperature; plasma-induced ellipticity and band averaging reduce observed soft-X-ray linear polarisation to 0-30%. Under typical X-ray pulsar accretion-channel conditions, thermal Comptonization leaves robust energy-dependent anisotropic signatures. Energy-resolved pulse profiles, pulsed fraction spectra, and polarisation thus provide complementary diagnostics of neutron star geometry, emission-region shape, and spectral formation.

astro-ph.HE↗

arXiv2026

A high-resolution X-ray view of the ultra-fast outflow in MAXI J1810-222

In previous work, it was reported that the Galactic black hole candidate MAXI J1810-222 exhibited a notable absorption spectral feature at around 1 keV in low-resolution X-ray spectra of CCD-like detectors. The feature was correlated with the spectral state of the source, being stronger in the soft states, as it occurs in the typical Fe K winds of X-ray binaries (XRBs). However, the results hinted towards rather extreme wind velocities of up to ~0.1 c. We therefore requested and obtained an observation with XMM-Newton to take advantage of the 10-fold higher spectral resolution (R ~200-400) provided by the RGS detector in order to resolve the lines and break the degeneracy between different models and interpretations. We applied state-of-the-art models of plasma in photoionisation equilibrium and multiphase interstellar medium. Further comparisons are performed with a re-analysis of NICER and NuSTAR data. The XMM-Newton/RGS spectrum is consistent with the presence of a mildly relativistic wind, confirming the earlier indications obtained with NICER, but places tighter constraints on the outflow properties, with the lines being intrinsically broad. The data would then favour magnetically driven winds, although thermal effects may still contribute to mass loading. NuSTAR and XMM-Newton (EPIC) show a further hotter component indicating a stratified or multiphase outflow. Fe K spectra taken with calorimetric detectors (e.g., Resolve on XRISM) will enable a high-resolution view of the complex extreme outflow in this source and shed new light on outflow processes in XRBs.

astro-ph.HE↗

arXiv2026

Unveiling the biconical geometry of the outflow in the ultraluminous X-ray source NGC 5204 X-1

Ultraluminous X-ray sources (ULXs) are non-nuclear X-ray binary systems that exceed the Eddington luminosity for a 10 Msun black hole. The majority of these sources are thought to be stellar-mass compact objects accreting at super-Eddington rates, exhibiting powerful relativistic winds. These winds have been identified through the detection of absorption lines with a blueshift as high as 0.3c and emission lines typically found at their laboratory wavelengths. In this work, we analysed the XMM-Newton data of the ULX NGC 5204 X-1, which has been observed to exhibit emission lines with a blueshift of about 0.3c. The aim of this study is to examine the geometry and physical properties of the accretion disc and the relativistic outflows. In addition, we aim to explore the factors that influence the ULX spectral transitions. We undertook an observing campaign with XMM-Newton to explore the source behaviour at different luminosities. In this first paper of the series, we performed high-resolution X-ray spectroscopy, including archival data, with the RGS instrument which allowed us to resolve both emission and absorption lines. The outflows features were characterised using physical models of plasma in collisional-ionisation and photoionisation equilibrium. We identify collisionally-ionised blueshifted and redshifted components at about 0.3c. These findings have high statistical significance and suggest a biconical structure for the outflow. Additionally, the analysis of the O VII line triplet observed in the spectrum enables us to infer physical properties of the low-velocity line-emitting plasma, e.g. electron density (ne $\sim 10^{10}$ cm$^{-3}$) and temperature (Te $ \geq 1.5 \times 10^5$ K). A hybrid plasma whose ionisation balance is affected by both collisions and radiation is favoured.

astro-ph.HE↗

arXiv2025

Double-hump spectrum, pulse profile dip, and pulsed fraction spectra from the low-accretion regime in the X-ray pulsar MAXI J0655-013

Accreting X-ray pulsars (XRPs) undergo different physical regimes depending on the mass accretion rate. Recent observations have shown a dramatic change in the emission properties of this class of sources observed at low luminosity. We explore the timing and spectral properties of the XRP MAXI J0655-013 observed in the low-luminosity regime (about 5x$10^{33}$ erg/s) to witness the corresponding spectral shape and pulse profiles. We employ recent $XMM$ and $NuSTAR$ pointed observations of the MAXI J0655-013 X-ray activity during the low-luminosity stage. We explore several spectral models to fit the data and test theoretical expectations of the dramatic transition of the spectral shape. We study the pulsating nature of the source and find a phase-connected timing solution. We explore the energy-resolved pulse profiles and the derived energy-dependence of different pulsed fraction estimators ($PF_{minmax}$ and $PF_{rms}$). We also obtain $NuSTAR$ pulsed fraction spectra (PFS) at different luminosity regimes. MAXI J0655-013 spectrum is well fitted by a double Comptonization model, in agreement with recent observational results and theoretical expectations that explain the observed spectrum as being composed of two distinct bumps, each dominated by different polarization modes. We measure a spin period of $1081.86\pm0.02$ s, consistent with the source spinning-up compared to previous observations, yielding an upper limit for the magnetic field strength of B<9x$10^{13}$ G. The pulse profiles show a single broad peak interrupted by a sharp dip that coincides with an increase in the hardness ratio. For the low-luminosity observation, the $PF_{minmax}$ increases with energy up to $\sim100\%$ in the 10-30 keV band, while the $PF_{rms}$ remains steady at $\sim60\%$. The PFS obtained at high luminosity shows evidence of an iron $Kα$ emission line but no indications of a cyclotron line.

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arXiv2025

X-ray spectral fitting with Monte Carlo Dropout Neural Networks

We present a novel approach using neural networks to recover X-ray spectral model parameters and quantify uncertainties, balancing accuracy and computational efficiency against traditional frequentist and Bayesian methods. Frequentist techniques often fall into local minima, compromising parameter estimation, while Bayesian methods, though more reliable, suffer from high computational costs. To address these challenges, we apply Monte Carlo Dropout within various neural network architectures trained on simulated spectra generated from a multiparameter emission model convolved with an instrument response. The model parameters are sampled from a predefined prior, and our proof of concept is illustrated using simulated data based on the NICER response matrix for simple emission models with up to five parameters. Our method delivers well-defined posterior distributions comparable to Bayesian inference, achieves accuracy akin to conventional spectral fitting, and is significantly less prone to local minima, thereby reducing the risk of selecting parameter outliers. Moreover, the approach improves computational speed by roughly an order of magnitude compared to traditional Bayesian techniques. This work demonstrates the potential of neural network-based methods as a robust alternative for X-ray spectral analysis, particularly in the context of future astronomical missions expected to generate extensive datasets.

astro-ph.IM↗

arXiv2025

A new pulsating neutron star in the Ultraluminous X-ray source NGC 4559 X7?

Ultraluminous X-ray sources (ULX) are extragalactic objects with X-ray luminosities above the Eddington limit for a 10 Msun black hole (BH). ULXs may host super-Eddington accreting neutron stars or stellar mass BH, although the exact proportion of the two populations is not yet known. We investigate the properties of the ULX NGC 4559 X7, which shows flux variability up to a factor of 5 on months-to-years and hours-to-days timescales. A flaring activity was also observed during the source highest flux epochs. Flares are unpredictable, with different durations and all flat-topped in flux. The latter suggests that, at the flare peaks, there is likely a common switch-off mechanism for the accretion onto the compact object. We analysed all the available XMM-Newton and Swift/XRT observations to investigate the spectral and temporal evolution of X7, looking for short and long-term variability. We look for long-term periodicities and for coherent signals through accelerated searches that included orbital corrections. We described the X7 spectra with two thermal components plus a cut-off powerlaw model. We found three well defined spectral states, where the spectral variability is mainly driven by the two harder components. In addition, a pulsed signal at 2.6-2.7s was detected in two XMM-Newton observations. The significance of these coherent signals is relatively weak but they are found in two different observations with the same parameter space for the orbital properties. If confirmed, it would imply a high spin-down of 1e-9 s/s, which could be extreme amongst the known pulsating ULXs. X7 would become a new extragalactic ULX pulsar. We discuss the spectral and temporal results of X7 in the context of super-Eddington accretion onto a stellar-mass compact object, in particular suggesting that the source might likely host a neutron star.

astro-ph.HE↗

arXiv2023

An ultrafast outflow in the black hole candidate MAXI J1810-222?

The transient X-ray source MAXI J1810-222 was discovered in 2018 and has been active ever since. A long combined radio and X-ray monitoring campaign was performed with ATCA and Swift respectively. It has been proposed that MAXI J1810-222 is a relatively distant black hole X-ray binary, albeit showing a very peculiar outburst behaviour. Here, we report on the spectral study of this source making use of a large sample of NICER observations performed between 2019 February and 2020 September. We detected a strong spectral absorption feature at $\sim$1 keV, which we have characterised with a physical photoionisation model. Via a deep scan of the parameters space, we obtained evidence for a spectral-state dependent outflow, with mildly relativistic speeds. In particular, the soft and intermediate states point to a hot plasma outflowing at 0.05-0.15 $c$. This speeds rule-out thermal winds and, hence, they suggest that such outflows could be radiation pressure or (most likely) magnetically-driven winds. Our results are crucial to test current theoretical models of wind formation in X-ray binaries.

astro-ph.HE↗

arXiv2022

Unveiling the disc structure in ultraluminous X-ray source NGC 55 ULX-1

Despite two decades of studies, it is still not clear whether ULX spectral transitions are due to stochastic variability in the wind or variations in the accretion rate or in the source geometry. The compact object is also unknown for most ULXs. In order to place constraints onto such scenarios and on the structure of the accretion disc, we studied the temporal evolution of the spectral components of the variable source NGC 55 ULX-1. Using recent and archival data obtained with the XMM-Newton satellite, we modelled the spectra with two blackbody components which we interpret as thermal emission from the inner accretion flow and the regions around or beyond the spherization radius. The luminosity-temperature (L-T) relation of each spectral component agrees with the L proportional T^4 relationship expected from a thin disc model, which suggests that the accretion rate is close to the Eddington limit. However, there are some small deviations at the highest luminosities, possibly due to an expansion of the disc and a contribution from the wind at higher accretion rates. Assuming that such deviations are due to the crossing of the Eddington or supercritical accretion rate, we estimate a compact object mass of 6-14 Msun, favouring a stellar-mass black hole as the accretor.

astro-ph.HE↗

arXiv2022

Galactic Observatory Science with the ASTRI Mini-Array at the Observatorio del Teide

The ASTRI (Astrofisica con Specchi a Tecnologia Replicante Italiana) Mini-Array will be composed of nine imaging atmospheric Cherenkov telescopes at the Observatorio del Teide site. The array will be best suited for astrophysical observations in the 0.3-200 TeV range with an angular resolution of few arc-minutes and an energy resolution of 10-15\%. A core-science programme in the first four years will be devoted to a limited number of key targets, addressing the most important open scientific questions in the very-high energy domain. At the same time, thanks to a wide field of view of about 10 degrees, ASTRI Mini-Array will observe many additional field sources, which will constitute the basis for the long-term observatory programme that will eventually cover all the accessible sky. In this paper, we review different astrophysical Galactic environments, e.g. pulsar wind nebulae, supernova remnants, and gamma-ray binaries, and show the results from a set of ASTRI Mini-Array simulations of some of these field sources made to highlight the expected performance of the array (even at large offset angles) and the important additional observatory science that will complement the core-science program.

astro-ph.HE↗

arXiv2022

A transient ultraluminous X-ray source in NGC 55

Ultraluminous X-ray sources (ULXs) are a class of accreting compact objects with X-ray luminosities above 10$^{39}$ erg s$^{-1}$. The average number of ULXs per galaxy is still not well constrained, especially given the uncertainty on the fraction of ULX transients. Here, we report the identification of a new transient ULX in the galaxy NGC 55 (which we label as ULX-2), thanks to recent XMM-Newton and the Neil Gehrels Swift Observatory observations. This object was previously classified as a transient X-ray source with a luminosity around a few 10$^{38}$ erg s$^{-1}$ in a 2010 XMM-Newton observation. Thanks to new and deeper observations ($\sim$ 130 ks each), we show that the source reaches a luminosity peak $>1.6 \times 10^{39}$ erg s$^{-1}$. The X-ray spectrum of ULX-2 is much softer than in previous observations and fits in the class of soft ULXs. It can be well described using a model with two thermal components, as often found in ULXs. The time scales of the X-ray variability are of the order of a month and are likely driven by small changes in the accretion rate or due to super-orbital modulations, attributed to precession of the accretion disc, which is similar to other ULXs.

astro-ph.HE↗

arXiv2022

Investigating the nature and properties of MAXI J1810-222 with radio and X-ray observations

We present results from radio and X-ray observations of the X-ray transient MAXI J1810-222. The nature of the accretor in this source has not been identified. In this paper, we show results from a quasi-simultaneous radio and X-ray monitoring campaign taken with the Australia Telescope Compact Array (ATCA), the Neil Gehrels Swift observatory X-ray telescope (XRT), and the Swift Burst Alert Telescope (BAT). We also analyse the X-ray temporal behaviour using observations from the Neutron star Interior Composition Explorer (NICER). Results show a seemingly peculiar X-ray spectral evolution of MAXI J1810-222 during this outburst, where the source was initially only detected in the soft X-ray band for the early part of the outburst. Then, ~200 days after MAXI J1810-222 was first detected the hard X-ray emission increased and the source transitioned to a long-lived (~1.5 years) bright, harder X-ray state. After this hard state, MAXI J1810-222 returned back to a softer state, before fading and transitioning again to a harder state and then appearing to follow a more typical outburst decay. From the X-ray spectral and timing properties, and the source's radio behaviour, we argue that the results from this study are most consistent with MAXI J1810-222 being a relatively distant ($\gtrsim$6 kpc) black hole X-ray binary. A sufficiently large distance to source can simply explain the seemingly odd outburst evolution that was observed, where only the brightest portion of the outburst was detectable by the all-sky X-ray telescopes.

astro-ph.HE↗

arXiv2021

Multiple accelerated particle populations in the Cygnus Loop with Fermi-LAT

The Cygnus Loop (G74.0-8.5) is a very well-known nearby supernova remnant (SNR) in our Galaxy. Thanks to its large size, brightness, and angular offset from the Galactic plane, it has been studied in detail from radio to $γ$-ray emission. The $γ$ -rays probe the populations of energetic particles and their acceleration mechanisms at low shock speeds. We present an analysis of the $γ$-ray emission detected by the Large Area Telescope on board the Fermi Gamma-ray Space Telescope over 11 years in the region of the Cygnus Loop. We performed detailed morphological and spectral studies of the $γ$-ray emission toward the remnant from 100 MeV to 100 GeV and compared it with X-ray, UV, optical, and radio images. The higher statistics with respect to the previous studies enabled us to decompose the emission from the remnant into two morphological components to model its nonthermal multiwavelength emission. The extended $γ$-ray emission is well correlated with the thermal X-ray and UV emission of the SNR. Our morphological analysis reveals that a model considering two contributions from the X-ray and the UV emission regions is the best description of the $γ$-ray data. Both components show a curved spectrum, but the X-ray component is softer and more curved than the UV component, suggesting a different physical origin. The multiwavelength modeling of emission toward the SNR suggests that the nonthermal radio and $γ$-ray emission associated with the UV component is mostly due to the reacceleration of preexisting cosmic rays by radiative shocks in the adjacent clouds, while the nonthermal emission associated with the X-ray component arises from freshly accelerated cosmic rays.

astro-ph.HE↗

arXiv2021

Tracking the evolution of the accretion flow in MAXI J1820+070 during its hard state with the JED-SAD model

X-ray binaries in outburst typically show two canonical X-ray spectral states, i.e. hard and soft states, in which the physical properties of the accretion flow and of the jet are known to change. Recently, the JED-SAD paradigm has been proposed for black hole X-ray binaries, aimed to address the accretion-ejection interplay in these systems. According to this model, the accretion flow is composed by an outer standard Shakura-Sunyaev disk (SAD) and an inner hot Jet Emitting Disk (JED). The JED produces both the hard X-ray emission, effectively playing the role of the hot corona, and the radio jets. In this paper, we use the JED-SAD model to describe the evolution of the accretion flow in the black hole transient MAXI J1820+070 during its hard and hard-intermediate states. Contrarily to the previous applications of this model, the Compton reflection component has been taken into account. We use eight broadband X-rays spectra, including NuSTAR, NICER and Swift data, providing a total spectral coverage of 0.8-190 keV. The data were directly fitted with the JED-SAD model. Our results suggest that the optically thick disk (i.e. the SAD) does not extend down to the ISCO in any of the considered epochs. In particular, as the system evolves towards the hard/intermediate state, we find that the inner radius decreases from $\sim$60 R$_{\rm G}$ in the first observation down to $\sim$30 R$_{\rm G}$ in the last one. This trend is accompanied by an increase of the mass-accretion rate. In all hard-intermediate state observations, two reflection components, characterized by different values of ionization, are required to adequately explain the data. These components likely originate from different regions of the SAD. We show that a flared outer disk could, in principle, explain the double reflection component.

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arXiv2021

The chameleon on the branches: spectral state transition and dips in NGC 247 ULX-1

Soft Ultra-Luminous X-ray (ULXs) sources are a subclass of the ULXs that can switch from a supersoft spectral state, where most of the luminosity is emitted below 1 keV, to a soft spectral state with significant emission above 1 keV. In a few systems, dips have been observed. The mechanism behind this state transition and the dips nature are still debated. To investigate these issues, we obtained a long XMM-Newton monitoring campaign of a member of this class, NGC 247 ULX-1. We computed the hardness-intensity diagram for the whole dataset and identified two different branches: the normal branch and the dipping branch, which we study with four and three hardness-intensity resolved spectra, respectively. All seven spectra are well described by two thermal components: a colder ($kT_{\rm bb}$ $\sim$ 0.1-0.2 keV) black-body, interpreted as emission from the photo-sphere of a radiatively-driven wind, and a hotter ($kT_{\rm disk}$ $\sim$ 0.6 keV) multicolour disk black-body, likely due to reprocessing of radiation emitted from the innermost regions. In addition, a complex pattern of emission and absorption lines has been taken into account based on previous high-resolution spectroscopic results. We studied the evolution of spectral parameters and the flux of the two thermal components along the two branches and discuss two scenarios possibly connecting the state transition and the dipping phenomenon. One is based on geometrical occultation of the emitting regions, the other invokes the onset of a propeller effect.

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arXiv2021

Swift/UVOT follow-up of Gravitational Wave Alerts in the O3 era

In this paper, we report on the observational performance of the Swift Ultra-violet/Optical Telescope (UVOT) in response to the Gravitational Wave alerts announced by the Advanced Laser Interferometer Gravitational Wave Observatory and the Advanced Virgo detector during the O3 period. We provide the observational strategy for follow-up of GW alerts and provide an overview of the processing and analysis of candidate optical/UV sources. For the O3 period, we also provide a statistical overview and report on serendipitous sources discovered by Swift/UVOT. Swift followed 18 gravitational-wave candidate alerts, with UVOT observing a total of 424 deg^2. We found 27 sources that changed in magnitude at the 3 sigma level compared with archival u or g-band catalogued values. Swift/UVOT also followed up a further 13 sources reported by other facilities during the O3 period. Using catalogue information, we divided these 40 sources into five initial classifications: 11 candidate active galactic nuclei (AGN)/quasars, 3 Cataclysmic Variables (CVs), 9 supernovae, 11 unidentified sources that had archival photometry and 6 uncatalogued sources for which no archival photometry was available. We have no strong evidence to identify any of these transients as counterparts to the GW events. The 17 unclassified sources are likely a mix of AGN and a class of fast-evolving transient, and one source may be a CV.

astro-ph.HE↗

arXiv2021

Origin and role of relativistic cosmic particles

This white paper briefly summarizes the importance of the study of relativistic cosmic rays, both as a constituent of our Universe, and through their impact on stellar and galactic evolution. The focus is on what can be learned over the coming decade through ground-based gamma-ray observations over the 20 GeV to 300 TeV range. The majority of the material is drawn directly from "Science with the Cherenkov Telescope Array", which describes the overall science case for CTA. We request that authors wishing to cite results contained in this white paper cite the original work.

astro-ph.HE↗

arXiv2021

Multi-messenger and transient astrophysics with the Cherenkov Telescope Array

The discovery of gravitational waves, high-energy neutrinos or the very-high-energy counterpart of gamma-ray bursts has revolutionized the high-energy and transient astrophysics community. The development of new instruments and analysis techniques will allow the discovery and/or follow-up of new transient sources. We describe the prospects for the Cherenkov Telescope Array (CTA), the next-generation ground-based gamma-ray observatory, for multi-messenger and transient astrophysics in the decade ahead. CTA will explore the most extreme environments via very-high-energy observations of compact objects, stellar collapse events, mergers and cosmic-ray accelerators.

astro-ph.HE↗